1. Field of the Invention
The present invention relates to a method of forming oxide or insulating films on semiconductor wafers and other substrates of different materials or single-crystal films (epitaxial films) having the same crystal orientation as that of the substrates thereon according to chemical reactions between reaction gases or chemical reactions of a reaction gas with the substrates, and an apparatus therefor such as an epitaxial growth apparatus, an oxidation apparatus, or the like, and more particularly, a method of forming a uniform thin film on each of a large number of substrates and an apparatus therefor.
2. Description of the Prior Art
In a conventional epitaxial growth apparatus (to be referred to as a first prior art apparatus hereinafter), a disk-like susceptor plate rotates around its vertical axis in a bell jar type reactor surrounded by an RF induction heater, and each substrate placed on the corresponding susceptor plate is heated to and kept at a high temperature (1,100 to 1,200.degree. C.) by the heater while a reaction gas is supplied to the surface of the substrate, thereby depositing a predetermined epitaxial growth film. In another conventional radiant heating-barrel type epitaxial growth apparatus (to be referred to as a second prior art apparatus hereinafter), a truncated pyramid susceptor is rotatably arranged in a cylindrical reactor, along its vertical axis, surrounded by an RF induction heating element and its RF induction coil, a plurality of semiconductor wafers or other substrates are placed on susceptor surfaces opposite to the heater, and a reaction gas is supplied to the reactor to cause predetermined epitaxial growth on the surfaces.
According to the first prior art apparatus, however, since the substrates are placed on a single plane perpendicular to the axis of the reactor, the number of substrates subject to epitaxial growth is inevitably limiting small. In addition, when the diameter of the wafer is increased, the utilization area of the susceptor is ineffectively decreased.
According to the second prior art apparatus, since the substrates can be placed on only the susceptor surfaces opposite to the heater, the number of substrates subject to epitaxial growth is still inevitably small. In addition, when the diameter of the wafer is incresed, the number of wafers to be epitaxially grown at a time is undesirably decreased. Furthermore, the substrate is positioned on the susceptor surface slightly inclined from a vertical line and it makes automatic loading/unloading difficult.
In either prior art apparatus, since the substrate is placed on the susceptor serving a heating element, at least part of a handling device must be brought into contact with the surface of the substrate when it is loaded/unloaded. Therefore, the substrate may be contaminated by this contact, and the yield tends to be reduced.
As shown in FIG. 7, in an apparatus (Japanese Unexamined Patent Publication (Kokai) No. 60-152675) (to be referred to as a third prior art apparatus), a substrate supporter 103 is arranged in a cylindrical reactor 102 surrounded by resistance heaters or other heat sources 101 in such a manner that the axis of the substrate supporter 103 coincides with that of the reactor 102. Substrates 104 are vertically stacked parallel to each other and substantially perpendicular to the axis of the reactor 102 in the supporter 103. A reaction gas is supplied from nozzles 105 mounted at the upper end portion of the reactor 102 and displaced to a gas exhaust port 106 at the lower end of the reactor 102, thereby performing predetermined epitaxial growth.
According to the third prior art apparatus, since the substrates 104 in the supporter 103 are vertically stacked to be parallel to each other and to be oriented in a direction substantially perpendicular to the axis of the cylindrical reactor 102, the number of substrates subject to epitaxial growth can be increased far beyond those of the first and second prior art apparatuses, so that a vertical thin-film formation apparatus suitable for a wafer having a large diameter can be provided. However, since the substrates are stacked in the axial direction of the reactor, an increase in the number of substrates results in a need for an increase in vertical dimension of heat sources arranged to surround the outer surface of the reactor. Temperature control in the reactor is thus difficult. As a result, a reaction gas temperature profile at the upper portion of the reactor becomes different from that at the lower portion thereof. Then, uniform thin films with the same quality cannot be formed on the upper and lower substrates.
Since the gas is supplied from the upper end of the reactor in a direction (i.e., axial direction) perpendicular to the surface of the substrate and is discharged from an exhaust port at the lower end of the reactor; an exhaust gas flow, a reactant in which is consumed by epitaxial growth on upper substrates continuously flows downward. In other words, when the gas flows downward, the concentration of the reactant is reduced, thereby causing nonuniform thicknesses of the films formed on the upper and lower substrates.
The decrease in concentration of the reactant is inversely proportional to an increase in concentration of a byproduct produced by epitaxial growth. In addition, since the gas temperature is increased, a difference in impurity concentration in the gas flow direction is enhanced. In other words, variations in resistivity tend to occur.
Still another conventional apparatus is proposed to increase the number of wafers simultaneously subject to epitaxial growth and simplify temperature control in the reactor. Unlike in the first and second prior art apparatuses, the susceptor is not heated in order to heat the substrate. A reactor wall is used as a hot wall to maintain the interior of the reactor at a high temperature, and a large number of substrates are horizontally or vertically stacked in the reactor, thereby performing epitaxial growth. This apparatus is called a hot wall type apparatus.
FIG. 8 shows an arrangement of a hot wall type apparatus. A gas conduit 114 axially extends in a reactor 113 along an exhaust port 112 formed at the lower end of a reaction chamber 111 and reaches the upper portion of the reaction chamber 111. A pair of right and left substrate boats 115 are arranged parallel to each other at the sides of the conduit 114 each to support horizontal arrays of a large number of vertically oriented substrates 116 along the widthwise direction of the reactor 113. Heating elements 119 are arranged to surround a reaction chamber 117 at its side and top and below a base 118 as the bottom surface of the reactor. The reactor 113 is maintained at a high temperature through the heating elements 119. A reaction gas is supplied to the reaction chamber 111 through the gas conduit 114 and passes through a large number of vertically oriented substrates 116 on the boats 115, thereby performing epitaxial growth on the surfaces of the substrates 116. The used gas is exhausted from the exhaust port located at the lower end of the inner chamber 113 (Japanese Unexamined Patent Publication (Kokai) No. 60-70177) (to be referred to as a fourth prior art apparatus hereinafter).
In a hot wall type apparatus of this type, however, the reactor itself which defines the reaction chamber is heated at a high temperature. In addition, the reactor is made of a material which includes the same material as the substrate, such as silicon or quartz. The reaction gas supplied to the reaction chamber chemically reacts with the inner wall surface of the reactor, and a chemical product is attached to the inner wall surface of the reactor. The product attached to the wall surface tends to peel from the wall surface due to a difference between the thermal expansion coefficients of the product and the wall material. Flakes peeling off from the inner wall surface tend to be planted to the surfaces of the substrates, resulting in defects.
According to the fourth prior art apparatus, a gas inlet port is located above the substrate, and the gas supplied from the inlet port is struck against the inner wall surface of the reactor above the substrates and then is dispensed around. The reaction byproducts tend to be attached to the upper portions of the side wall surfaces of the reactor, and the defects described above are further accelarated.
According to the system of the fourth prior art apparatus in which the gas is distributed in the reaction chamber from the single gas inlet port, a gas turbulence tends to be formed. In addition, since the substrates are located immediately below the gas inlet port, the gas turbulence affects directly the surface of the substrate. As a result, a uniform film thickness profile cannot be obtained.
In the fourth prior art apparatus, the sealed housing is located to surround the reactor and the reaction chamber is maintained at a predetermined temperature. Since the reaction gas is supplied to the reactor from the gas conduit at room temperature, a temperature difference between the reaction gas and the interior of the reactor is large although the reaction gas is preheated. A nonuniform temperature profile occurs in the reaction region on the surface of the substrate when the substrates are located immediately below the gas inlet port in the construction of the fourth prior art apparatus. As a result, a uniform film thickness, a uniform film quality, and a uniform resistivity distribution cannot be obtained.
In the apparatus for performing epitaxial growth while the reaction gas is supplied from the upper portion of the reaction chamber in which the substrates are vertically oriented, it is difficult to obtain a uniform film thickness distribution and a uniform resistivity distribution. This apparatus can be used for forming an insulating film by thermal oxidation of silicon. However, it cannot be used as an apparatus for forming an epitaxial film with high precision in thickness from the technical viewpoints.